Hi Matt:
Conventional wisdom says to charge any unknown battery at 10% of its
ampere hour rating for 150 to 160% of the discharge rate. I have found
this to be pretty conservative and low risk (though not always
convenient). For your 1.3 amp-hour gel cell, this works out to 130 mA for
15 to 16 hours. If, at the end, you don't have a no-load voltage of
something like 13 VDC, then it probably is a sick puppy anyway. I don't
believe gel cels respond to "exercise" like nicads do, so it would be the
end of the road for that particular pack.
As far as charge circuits go, "KISS" rules my domain. I was fortunate
enough to pick up a nice Lambda 0-48 VDC 3 Amp adjustable power supply
some years ago at a flea market, so I simply series up the supply, a large
resistor, my VOM on its mA scale, and the battery. I tweak the supply
until the charge current is 130 mA (for your example), then walk away. It
is desirable to use as large a resistor as possible (and lots of voltage),
as that ensures that the charge current is more or less constant. It is a
good idea to check the lash-up several times over the 15 hour period, and
to disconnect it promptly once done. If you are prone to forgetting these
kinds of things, a cheap security timer might be a good add-on. A series
diode mght be worthwhile, too, as some power supplies have no sense of
humour about being reverse charged.
Good luck!
John
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